Question

Would you expect \(\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{O}(g)\) to have a positive or a negative \(\Delta H ?\) Explain.

Short answer

The \(\text{ΔH} \) is positive because breaking chemical bonds requires energy input.

Step-by-step solution

- Understand the Reaction

The reaction given is the dissociation of diatomic oxygen \(\text{O}_2 \) into two individual oxygen atoms \(\text{O}\). This process breaks the oxygen-oxygen double bond in \(\text{O}_2 \).

- Consider Bond Breaking and Energy

Breaking chemical bonds requires energy input. When \(\text{O}_2 \) dissociates, energy must be supplied to break the bond between the two oxygen atoms. This energy input increases the enthalpy of the system.

- Determine the Sign of \(\text{ΔH}\)

Since energy is required to break the \(\text{O}_2 \) bond, \(\text{ΔH}\) for the reaction is positive. The positive \(\text{ΔH}\) indicates that the process is endothermic.

- Summarize the Conclusion

Hence, given that breaking the bond requires energy, the enthalpy change \(\text{ΔH}\) for the dissociation of \(\text{O}_2 \) into two oxygen atoms is positive.

Key concepts

Bond Dissociation

When we talk about breaking chemical bonds, we refer to bond dissociation. In the case of the reaction \(\text{O}_2 \rightarrow 2 \text{O}\), bond dissociation specifically involves breaking the oxygen-oxygen double bond. This process transforms one molecule of diatomic oxygen \(\text{O}_2\) into two separate oxygen atoms \(\text{O}\).
Bond dissociation requires energy. This energy is used to overcome the attractive forces holding the two oxygen atoms together in the \(\text{O}_2\) molecule. Without providing this energy, the bond cannot be broken.
An easy way to visualize bond dissociation is to imagine breaking a stick. You need to exert effort (energy) to break it into two pieces. Similarly, breaking a chemical bond needs energy input from an external source.

Enthalpy Change

Enthalpy change, represented by \(\text{ΔH}\), is a measure of the heat energy absorbed or released during a chemical reaction. When the enthalpy change \(\text{ΔH}\) is positive, it indicates an endothermic reaction, meaning the system absorbs energy.
In our reaction \(\text{O}_2 \rightarrow 2 \text{O}\), we are breaking a bond, which requires energy. As a result, the enthalpy of the system increases. This increase in enthalpy tells us that energy is taken in from the surroundings, making \(\text{ΔH}\) positive.
Think of enthalpy change as a way to track the energy flow in and out of a reaction. A positive \(\text{ΔH}\) is like a bank transaction where money (energy) is deposited into your account, indicating a gain in energy.

Energy Input

Energy input is the energy that needs to be supplied to initiate a reaction. For the dissociation of \(\text{O}_2 \rightarrow 2 \text{O}\), energy input is needed to break the oxygen-oxygen bond.
Without this energy input, the \(\text{O}_2\) molecule remains stable and intact. The energy input acts as a catalyst that drives the dissociation forward, making it possible to convert diatomic oxygen into individual oxygen atoms.
Consider energy input like the fuel required to start a car; without fuel, the car won't run. Similarly, without the necessary energy input, the \(\text{O}_2\) molecule's bond won't break, and the dissociation reaction won't occur.